Marking pen



FIG. 1 is a top, front, left perspective view, on a slightly enlarged scale, of a marking pen showing my new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a left side elevational view thereof, the right side being the same in appearance as the left side;

FIG. 4 is a rear elevational view thereof;

FIG. 5 is a top plan view thereof; and,

FIG. 6 is a bottom view thereof. 

The ornamental design for a marking pen, as shown and described. 